A multiplexer is frequently connected upstream from an analog/digital converter for the purpose of converting analog signals from multiple inputs into corresponding digital signals with the aid of a single analog/digital converter. A possible error of the multiplexer, such as a short circuit to another potential, a coupling of multiple channels among one another or an interruption in one of the channels, is not recognized. In order to be able to use such an analog/digital converter system including a multiplexer and an analog/digital converter for safety-relevant variables, a check for reliability is required. The analog/digital converter system may, for example, be configured as a part of a sensor system which may be implemented as an ASIC (application-specific integrated circuit).
The presently common methods carry out the check of the multiplexer, if at all, with the aid of a plurality of instructions and a software program which is processed by a microcontroller in a corresponding control device. Here, the program sequence must then still wait for the operations of the ASIC to be processed before the next program step for checking may be initiated. This results in a great software complexity and in long waiting times of the microcontroller, which often remain unused, during the test phase.
In the unexamined patent application DE 10 2008 042 400 A1, for example, a method and a correction device for correcting an offset error of an analog/digital converter system are discussed. The correction device described above includes a multiplexer for connecting an input of an analog/digital converter to at least one measuring channel or an offset channel. The offset channel provides a predefined reference voltage level or a predefined reference current value. An arithmetic unit stores in a memory a value which is provided by an output of the analog/digital converter as the offset value or links a value which is provided by the output of the analog/digital converter as a measured value to the stored offset value on the basis of a linking structure and provides the obtained value as the corrected measured value to a transmission unit. Furthermore, a control unit is provided which switches the multiplexer and the arithmetic unit into a first or a second operating mode, in the first operating mode, the input of the analog/digital converter being connected to the offset channel and the value provided by the output of the analog/digital converter being interpreted in the arithmetic unit as an offset error value, and in the second operating mode, the input of the analog/digital converter being connected to a measuring channel and the value provided by the output being interpreted in the arithmetic unit as a measured value.